At present radio communication systems such as mobile phone systems and radio LANs (Local Area Networks) are widely used. In some radio communication systems a transmission apparatus uses a plurality of transmission antennas in parallel for transmitting data to a receiving apparatus. With a mobile phone system based on a standard referred to as LTE (Long Term Evolution), for example, a radio base station can use a maximum of four transmission antennas in parallel. In addition, with a mobile phone system based on a standard referred to as LTE-A (LTE-Advanced) corresponding to the next generation of LTE, the possibility that a radio base station can use a maximum of eight transmission antennas in parallel is discussed (see, for example, 3GPP (3rd Generation Partnership Project), “Requirements for Further Advancements for E-UTRA”, TR 36.913, V8.0.0, June 2008).
If a plurality of transmission antennas are used, a transmission apparatus may transmit a known signal referred to as a reference signal (RS) from each transmission antenna (see, for example, 3GPP (3rd Generation Partnership Project), “DL RS Designs for Higher Order MIMO”, TSG-RAN WG1 #56, R1-090619, February 2009; 3GPP (3rd Generation Partnership Project), “DL Reference Signal Design for 8×8 MIMO in LTE-Advanced”, TSG-RAN WG1 #56, R1-090706, February 2009; and 3GPP (3rd Generation Partnership Project), “An Efficient Reference Signal Design in LTE-Advanced”, TSG-RAN WG1 #56, R1-090949, February 2009). A part of a radio resource is assigned for transmitting the known signal. A receiving apparatus detects the known signal included in a received signal. By doing so, the receiving apparatus can estimate a channel state for each transmission antenna. The result of estimating the channel state can be used for data demodulation, feedback of communication quality to the transmission apparatus, or the like.
By the way, if a transmission apparatus includes transmission antennas the number of which exceeds a maximum number which a receiving apparatus can detect, the problem of a transmission control method arises. In order to solve this problem, a control method referred to as virtual antenna mapping is proposed (see, for example, 3GPP (3rd Generation Partnership Project), “Downlink Multiple Antenna Schemes for LTE-Advanced”, TSG-RAN WG1 #54, R1-083228, August 2008; and 3GPP (3rd Generation Partnership Project), “Discussion on RS Design for High-order MIMO in LTE-A”, TSG-RAN WG1 #56, R1-090751, February 2009). With this control method two transmission antennas, for example, are treated as a pair. The two transmission antennas which form a pair transmit known signals by the use of the same resource region (at the same frequency and the same timing, for example) and transmit data which are equal in content (transmit data by CDD (Cyclic Delay Diversity), for example). As a result, a receiving apparatus recognizes the two physical transmission antennas as one virtual transmission antenna.
It is assumed that a transmission apparatus performs radio communication with plural types of receiving apparatus which differ in the maximum number of detectable transmission antennas. For example, a transmission apparatus includes eight transmission antennas and performs radio communication with both of a receiving apparatus which can detect a maximum of four transmission antennas and a receiving apparatus which can detect all the eight transmission antennas.
In this case, a known signal transmission method is problematic. One possible method is to separately transmit known signals according to receiving apparatus type. For example, both of known signals for detecting the eight transmission antennas and known signals for recognizing the eight transmission antennas as four virtual transmission antennas are transmitted. With this method, however, radio resources secured for transmitting known signals expand and radio resources used for transmitting data reduce. In the above example, eight resource regions corresponding to the eight transmission antennas and four resource regions corresponding to the four pairs may be secured.